The invention relates to an implantable device for determining intracranial pressures, wherein a pressure measuring device is used, which is operatively connected to a sensor for a telemetric measured value transfer.
The determination of intracranial pressure assumes an extremely important role in neurosurgical procedures. The most precise and simple minimally invasive measurement of intracranial pressure possible counts to this day as one of the aims of medical technology that has not been satisfactorily resolved.
It has been the object of numerous inventions to solve this problem. However, all the systems available hitherto have serious disadvantages, and to overcome these is the object of the invention presented here.
Invasive measurement methods have prevailed in clinical practice, in which a sensor is inserted into the body, wherein the signal is passed via a cable connection to an external device for display and evaluation of the measured value. This frequently occurs in combination with an artificial drainage pipe, through which brain fluid is to be drained out of the body. The very high risk of infection is critical with such systems. Long procedures can only be conducted by very expensive prophylaxis against infections and with multiple replacements of the pressure sensor. However, especially in the case of patients with hydrocephalus, it is the progress of the intracranial pressure in an out of clinic situation after implantation of an internal artificial drainage system that is of high diagnostic interest. Systems, which pass the measured signal through the intact skin or allow measurement through the skin, are suitable for such tasks.
Patent DE 196 38 813 C1 describes an implantable pressure sensor, which is connected to flexible foil strip conductors and is surrounded by a substrate in the region of the sensor element, which has a higher mechanical strength than the foil strip conductor and which is enveloped together with the sensor element into a flexible body. The structure should make a reliable and inexpensive measuring device possible, which does not, however, enable any reduction with respect to the risk of infection because of the necessary penetration of the skin. In association with the sensor technology, reference is made to patent U.S. Pat. No. 4,738,267, in which a plastic capsule with a membrane is used, to which a strain gauge is attached. The imbalance of the Wheatstone bridge is interpreted as the magnitude for the existing pressure. Such a sensor operates imprecisely and exhibits an unacceptably high drift behaviour. For this reason, it has not prevailed as implantable intracranial pressure sensor.
The same technique is also described for telemetric determination of intracorporeal pressures in patent application DE 197 05 474 as an application for a patent of addition to DE 196 38 813 C1. However, there are no indications given here as to how biocompatibility is to be assured. Such a sensor has so far not advanced to the commercial stage.
A likewise telemetric method for determining intracranial pressure is described in patent U.S. Pat. No. 6,113,553. The claims applied for here relate to measurement that is as drift-free and stable in the long term as possible with description of the electronic structure. A capacitive structure is used here, wherein the sensor is to be embedded in the bone of the patient. This is necessary because of the bulky structure of the sensor. The extent to which the actual properties of the sensor design meet the high requirements for intracranial pressure measurement with respect to accuracy and drift behaviour is not known, since such a sensor is not as yet commercially available and therefore could not be subjected to any independent tests.
A method for determining intracranial pressure without skin penetration is likewise described in patent U.S. Pat. No. 4,676,255 from 1987. The idea here was to use the principle of relaxed membranes. A sensor placed under the skin has its zero position as long as intracranially in relation to the surrounding area no positive or negative differential pressure is present. When the intracranial pressure rises or falls, the sensor moves out of the zero position. The precise pressure that is necessary to bring the sensor into the zero position again is now applied through the skin. The pressure necessary for this should then correspond to the intracranial pressure. This method has not been able to prevail clinically. Reasons for this are the variance of skin from patient to patient, the technically difficult and imperfect determination of the zero position and also the complicated generation of the necessary external pressure pad.
Substantially better possibilities are offered by telemetric approaches, in which an extremely small pressure sensor is inserted into the body, which is connected by means of cable likewise inserted into the body to a coil, by means of which, on the one hand, the sensor can be supplied with energy where required and, on the other hand, the measured signal can be transmitted to the outside to a receiver unit for further processing.
A method is described in patent DE 198 58 172 that determines the intracorporeal pressure directly by means of a sensor element using microsystems technology. This centres on the determination of the internal pressure of the eye. The implant should therefore be as small and light as possible. The coating of the sensor is of decisive importance when using this technology to determine intracranial pressure. Such a sensor is described in patent DE 101 56 494, wherein a metal layer as well as a biocompatible plastic layer is provided at least in sections to assure biocompatibility. Such a structure has considerable disadvantages. A coating of the sensor element of whatever type permits impairment of the measurement because of the penetration through this very layer, the properties of which can change over time. The layer can be damaged as a result of the actions of forces from the outside. A drift behaviour can also be problematic as a result of ageing, in particular of the plastic layers.
In order to ensure a homogeneous and secure transfer of the pressure prevailing around the sensor, a technique is described in patent EP 1 312 302 A2, in which a medium arranged around the sensor is surrounded by a flexible sheath. How the biocompatibility of the flexible sheath is to be assured is not described in the patent document. The favoured use of silicone oil in the application for optimum transfer of the existing pressure appears problematic taking into consideration the risk factors.